1. Field of the Invention
The present invention relates to an apparatus and method for loading a mask into a lithographic system employed in the fabrication of integrated circuits in semiconductor wafers or other substrates and an improved cassette usable with the apparatus. More particularly, the improved apparatus and cassette are useful in manufacturing semiconductor devices where X-ray printing circuit geometries to 0.5 micrometer or smaller are desired. X-ray interfering particle generation must be kept to a minimum when loading or unloading the mask into the lithography apparatus and in removing and replacing the mask in a cassette so as to prevent anomalies in the X-ray processing which utilizes the mask.
2. Description of the Prior Art
Heretofore, cassettes have been employed to store masks and allow their automatic removal. For example, a mask has been mounted on a flat tray and a cover placed thereover and held to the tray with locking fingers. means have been provided to open the locking fingers and to raise the cover to allow mechanical fingers on a transport arm to physically grasp the edges the of mask and move it to a vacuum holder on the photolithography apparatus. Such a system often results in relative rubbing between the mask and tray in storage, handling, unloading or loading, and between the mask and the mechanical fingers, resulting in the generation of very fine particles from the mask, tray or fingers. These fine particles if present on the operable printing portion of the mask, may be absorbive of X-ray eminations resulting in subsequent misprinting of images on the semiconductor wafer. This diminishes integrated circuit yields from that wafer. The prior art mask transport and cassette system have not afforded the cleanliness necessary for the exacting specifications in this field. The prior art further has not included calibration means operable in the same transport system for initially setting the gap between a mask aligner and the wafer and which clearly informs a user whether a mask or calibration disc is being loaded.